Conditions inside a unit of a youth jail are so filthy and "squalid" that it should be immediately demolished, according to the chief inspector of prisons.

Anne Owers says in her inspection report on Portland young offenders' institution, published today, that the old building that houses the 72 prisoners in Rodney unit is insanitary, unfit for use and should be promptly closed.

The report on the remote Dorset jail says it holds 580 young adults aged 18 to 21, mostly in old and forbidding buildings.

More than 40% of Portland's population come from black or minority ethnic backgrounds but almost none of the staff. However, Owers says efforts have been made to bring in black role models to assist in sentence planning and activities.

The inspectors say that most of the units in Portland, particularly the newer ones, are in good condition. "However there remained one unit, Rodney, with no integral sanitation, where conditions can only be described as squalid: breaching acceptable standards of health and safety and in general unkempt and uncared for.This unit urgently needs to close, as its sister unit has already done."

Rodney unit, which has three floors, and cells on four landings on the two upper levels, has no in-cell sanitation. Some toilets and sinks in the communal areasare dirty and access at night is controlled by a computer system that allows up to four visits a night for a maximum of 10 minutes, the inspectors say. Only one prisoner is allowed out at a time. A prisoner who returns late to his cell is denied further access that night by the computer.

Phil Wheatley, the director general of the National Offender Management Service, said Rodney unit would be decommissioned shortly. "The director of offender management has already taken the worst accommodation out of use," he said.